Cathode ray tube apparatus

ABSTRACT

A cathode ray tube apparatus includes an electroluminescent viewing screen, an electron beam source, a plate-like electronbeam-directing electrode spaced from the screen surface, and means to connect a static potential between the screen surface and the plate-like electrode to produce a field in the space between them for directing an electron beam obtained from the source on to the screen, such that when the electron beam is subjected to orthogonal scanning before it is launched into the space at varying angles to the normal beam axis it is so directed within the space by the field that it is scanned orthogonally over the screen surface.

United States Patent McCarthy et a1.

[ 1 June 17, 1975 CATHODE RAY TUBE APPARATUS Inventors: Brian Dennis McCarthy, Oxted;

Anthony Victor Krause, Ashtead, both of England [21] Appl. No.: 129,824

3,215,888 11/1965 Haun et a1. 315/21 R 3,377,500 4/1968 Burns 315/14 3,435,277 3/1969 Haun et a1. 313/42 3,461,333 8/1969 Haun 315/23 X 3,531,681 9/1970 Harden, Jr, 315/13 Primary Examiner-Benjamin R. Padgett Assistant ExaminerP. A. Nelson Attorney, Agent, or Firm-Louis Etlinger; Joseph E. Funk ABSTRACT [30] Foreign Application Priority Data [57] A r. 2, 1970 United Kingdom 15746/70 A cathode ray tube apparatus includes an electroluminescent viewing screen, an electron beam source, a [52] US. Cl. 315/366 plate-like electron-beam-directing electrode spaced [5 l 1 Int. Cl. H0lj 29/70 from the screen surface, and means to connect a static [58] Field of Search 315/23, 366 potential between the screen surface and the plate-like electrode to produce a field in the space between [56] References Cited them for directing an electron beam obtained from the UNITED STATES PATENTS source on to the screen, such that when the electron 2,972,703 2/1961 Aiken 315/10 Orthogonal. g 2,997,621 8/1961 Schlesinger l 315/356 e "9 I 9 'F es 9 2'99995-7 9H9 Schagen a a] I I l I 313/79 mal beam axls it is so d1rected withm the space by the 3,068,360 12/1962 Nicoll 250/213 field that it is scanned orthogonally over the screen 3,118,084 l/1964 Haun et al. 315/356 surface.

3,171,056 2/1965 Gabor 315/356 3,181,027 4/1965 Geer 315/13 3 Clams, Figures PATENTEIJJUN 17 ms SHEET INVENTORS E WAD/WW. mm m R O wxmw m A mm 0 M M HN BA PATENTEDJUN 17 ms saw 2 4O 42 44 46 FM FM m P 205 2 Focus FOCUS /6 N99 206 UNEARITY OECSDER CORRECTION 60:22am I92 F' .20!

x x DEFLN gases-"24"- /A AME I96 -198 2 '93 Y DEFL'N i G ER D/A AME RE lsr r208 LINEARITY W050 (200 CORRECTION AME Y DECODER ASTG. .2109 I94 CORRECTION INVENTORS BRIAN D. MCCARTHY ANTHONY V. KRAUSE BY 7/ n g ATTORNEY PATENTEDJUN 17 ms SHEET IOI 9| kw FIG. /0

INVENTORS BRIAN D. MCCARTHY ANTHONY v. KRAUSE BY 2M 2 ATTORNEY PATENTEI] JUN 17 5 SHEET PATENTEDJUN 17 I975 sum 6 INVENTORS BRIAN D. McCARTHY ANTHONY V. KRAUSE BY ATTORNEY I l X PATENTEDJUN 1 7 ms SHEET BRIAN D. MCCARTHY ANTHONY V. KRAUSE FIG. /5

ATTORNEY CATHODE RAY TUBE APPARATUS FIELD OF THE INVENTION This invention relates to cathode ray tube apparatus which can be used to provide visible displays, either in black and white or in colour, and to cathode ray tube apparatus which acts as a camera pick-up tube. A feature of cathode ray tube apparatus according to the present invention is that it has a smaller back to front dimension for a given area of display or image production than have those commonly in commercial use and it thus enables a display or image to be provided in a space with a limited depth. The apparatus is particularly suitable for use in providing displays on aircraft control panels, though its use in not limited to this application.

BACKGROUND OF THE INVENTION Cathode ray tube apparatus which have a smaller back to front dimension than those commonly used have previously been proposed and have been referred to as flat"cathode ray tube apparatus. However, none of the previously proposed apparatus has, so far, been brought into general commercial use.

Examples of such previously proposed flat cathode ray tube apparatus are described in U.S. Pat. Specifications Nos. 2,795,729 and 2,795,731. These previously proposed arrangements required an array of parallel electrodes behind the viewing screen to control the scanning of the electron beam in one direction and were either complicated to manufacture or needed comparatively high voltages to be switched and applied to respective electrodes in the array during a scanning operation.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a novel and improved cathode ray tube apparatus.

An additional object of this invention is to provide a cathode ray tube apparatus which is comparatively simple to manufacture and operate.

It is a further object of this invention to provide a flat cathode ray tube apparatus which employs, to a large extent, known electron beam deflection systems and electronoptical focussing and accelerating systems.

Briefly, in the cathode ray tube apparatus of the invention a space for a beam directing field is provided between a surface of a viewing screen and a beam directing electrode and a beam directing field, which is produced in the space, is such that an electron beam which is deflected into a given trajectory before it enters the space is directed to a discrete spot on the screen corresponding to the particular trajectory.

BRIEF DESCRIPTION OF THE DRAWINGS The above-mentioned and other features and objects of the invention will become more apparent upon reference to the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a diagrammatic front view of a cathode ray tube,

FIG. 2 is a section on the line lIlI of FIG. 1,

FIG. 3 is a side view of a magnetic lens assembly shown in FIGS. 1 and 2,

FIG. 4 is a diagrammatic section through an envelope similar to that of FIG. 2 showing diagrammatically a beam directing field,

FIG. 5 is a plan view of a beam directing electrode,

FIG. 6 is a diagrammatic section of a part of a twocolour cathode ray tube,

FIGS. 7, 8, and 9 are diagrammatic sections of alternative cathode ray tube arrangements which may be used for colour,

FIG. 10 is a diagrammatic section of a further screen and beam directing arrangement,

FIG. 11 is a cut-away perspective view ofa demountable experimental type cathode ray tube,

FIG. 12 is a diagrammatic section through a modified cathode ray tube,

FIG. 13 is a diagrammatic front view of a further cathode ray tube arrangement,

FIG. 14 is a diagrammatic front view of a particular embodiment of a cathode ray tube,

FIG. 15 is a section on the line X-X of FIG, 14 and FIG. 16 is a schematic diagram of an electrical circuit for use with a cathode ray tube of the type shown in FIGS. 14 and 15.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In FIGS. 1 and 2 there is shown a cathode ray tube having an envelope 1 through which there is visible a planar transparent support screen 2. On the inner face of the support screen there is an electroluminescent screen 3 which comprises a transparent conducting layer and a coating of material which luminesces when bombarded by electrons. Directly behind and spaced from the screen 2 there is a beam-directing electrode 4 which is in the form of a single sheet, though it may be made from a plurality of electrically interconnected sheets. The support screen 2 upon which the luminescent material is provided may alternatively be the transparent face of the envelope 1. In a further modification, the beam directing electrode is made of a transparent material and the screen may be viewed from either or both sides of the envelope. The beam directing electrode 4 is of greater height in the normal direction of the beam than the screen 2 and along the edges of the electrode 4 extending beyond the area of the screen 2 there are, as shown in FIG. 1, strips of electrically conducting material 5 and 6. Terminals 7 and 8 on the envelope 1 are electrically connected to the strips 5 and 6. A uniform resistive layer is applied to the electrode 4 between the strips 5 and 6. The conducting layer of the electroluminescent screen 3 is electrically connected to a terminal 9 on the envelope 1.

The envelope 1 has a neck 10 which houses an electron gun 11 and orthogonally operating electrostatic deflecting electrode systems 12 and 13. An electromagnetic collimator lens structure 14 is positioned around the flared part of the envelope. Other correcting lens systems may be provided as required. An electron beam, indicated in two positions by solid and dotted lines 15 and 16 respectively, is shown impinging upon the screen at points distant from and near to the beam source ll.

As may be seen from FIG. 3 the collimator lens 14 includes a pair of magnets 17 and 18 joined by a pair of pole pieces 19 and 20 arranged to provide a ring magnet assembly.

In operation, biassing potentials, with respect to the E.H.T. potential applied to the electroluminescent screen 3 via the terminal 9, are applied between the terminals 7 and 8 and connected across the electrode 4 between the strips 5 and 6. The biassing potential on the strip 5 is of a higher value than that on the strip 6 and an electric field is produced in the space 21 (FIG. 2) between the beam directing electrode 4 and the screen 3 having an intensity which is greatest in the part of the region of the space 21 furthest from the electron gun 11. The uniform layer of resistive material between the strips 5 and 6 causes the beam directing biassing potential to be so distributed over the electrode 4 that the field in the space 21, though static, is non-uniform and varies in intensity substantially linearly throughout the space 2] in the direction from the strip 6 to the strip 5, reaching a maximum value in the region of the space 21 extending between the edge of the screen 3 remote from the source 7 and the strip 5.

Deflection potentials are applied to the deflecting electrode systems 12 and 13 in a known manner to cause the electron beam emitted by the gun 11 to be deflected in two orthogonal directions and to set up a raster or other selected pattern before it enters the space 21. When the electron beam is given a lateral component of movement from the undeflected path before it enters the beam directing field space 21 between the screen 3 and the electrode 4, its direction of entry into the space changes and the distribution of the beam directing field in that space is such that the beam is caused to be directed, as it passes through the space 21, towards a given position on the screen 3 according to the angle through which it is moved laterally from its undeflected path before it enters the space 21, or in other words according to its direction of entry into the space 21. The electron beam may initially be directed towards the electrode 4 as it enters the space 21 in order eventually to impinge upon certain positions on the screen 3. The actual position in which the beam impinges upon the screen 3 is determined by the angle through which it is deflected before it enters the beam directing field, the velocity ofthe beam and the distributed intensity of the field in the part of the space 21 through which it passes.

The deflection fields set up by the systems 12 and 13 determine the angle at which the beam enters the region 21 and by suitably shaping these fields, the shape of the area of the screen 3 scanned by the beam may be controlled. The collimator lens structure 10 is designed to correct for the form of distortion commonly known as keystone" distortion in which the width of a display nearer to the source 11 would otherwise be less than the width of the display further from the source 7. However, as has been indicated above, keystone distortion may alternatively be corrected by suitably shaping the deflection fields for example by interchanging the electrode systems l2 and 13 and shaping the electrodes of the system 12 to provide the required correcting field.

A scanning roster may be set up by the electrode systems l2 and 13 in such a way that the scanning lines developed on the screen 3 are either in planes substantially at right angles to the axis of the electron beam emerging from the gun I] or parallel to this axis.

Instead of setting up a raster pattern the beam may be deflected by the systems 12 and 13 to write alphanumeric information directly on to the screen. This latter form of display is particularly suitable for use in displaying quantative information, for example when monitoring information on an aircraft control panel. It is necessary in modern aircraft to monitor a large number of different sources of information and, since these sources can be scanned electrically comparatively easily, information exceeding given limits from any of the sources may be slected and written on the separate parts of the display simultaneously, in order to highlight the fact that a limit has been exceeded.

Referring to FIG. 4 there is shown a section through a part of a glass envelope 25 having a viewing face 26 and an electroluminescent layer 27 deposited upon a conducting layer 28 formed directly on the inside of the other face 29 of the envelope and forming a viewing screen 30. On the inside of the face 26 there is a transparent uniform resistive coating 31 formed by depositing a thin layer of oxidized chromium. The screen 30 is viewed through the coating 31. Alternatively, the conducting layer 28 can be of a transparent material and an image on the screen 30 can be viewed through the face 29.

Terminals 32 and 33 on the envelope 25 are electrically connected to the opposite edges of the coating 31 via respective conducting strips 34 and 35. The coating 31 forms a beam directing electrode. The conducting layer 28 is electrically connected to a terminal 36. An electron gun (not shown) is housed, together with other beam controlling elements, in the neck 37 of the envelope.

in operation, an E.H.T. voltage is connected to the terminal 36 and biassing voltages, which are lower than that of the E.H.T., but which are positive with respect to that of the electron-beam source and have a potential difference between them are connected to respective terminals 32 and 33. The voltage connected to the terminal 32 is greater than that connected to the terminal 33 so that the gradient of the field intensity across the space between the screen 30 and the beam directing electrode 31 is smaller nearer to the gun and increases with distance from the gun. Such an arrangement produces a field which increases linearly in intensity with distance from the gun along the length of the envelope, as indicated by the lines 34g. The relative voltages might, for example, be an E.H.T. of 20 KV connected to the terminal 36, and biasing potentials of l5 and l0 kV obtained from the same source applied to the terminals 32 and 33 respectively. Alternatively, a field of linearly increasing intensity with distance from the gun can be provided by depositing the electroluminescent layer 27 on the uniform resistive coating 31 to form the viewing screen. The conductive layer 28 can than be employed as the beam directing electrode. In the operation of this embodiment a voltage difference is applied across the resistive coating 31 via the two terminals 32 and 33 to provide the E.H.T. and a biassing potential of lower value is applied to the conductive layer 28 forming the beam directing electrode via the terminal 36. In this embodiment the E.H.T. potentials on the terminals 32 and 33 are chosen with respect to the biassing potential on the beam directing electrode formed by the layer 28 in such a way that a smaller gradient of field intensity 34 is provided across the space between the screen and the beam directing electrode as the electron gun is approached along the space.

Although in this embodiment the screen and beam directing electrode are deposited directly on the inner faces of the envelope, either or both of them may be formed on supports which are then introduced into the envelope before its manufacture is complete.

Referring to FIG. 5 there is shown diagrammatically a plan view of a beam directing electrode which is designed to provide a non-linear beam deflecting field and which can be used in place of electrode 4 of FIG. 1. The electrode of FIG. 5 has an insulating support sheet 38 across which there are provided conductive strips 39 arranged on parallel axes in planes which are at right angles to the surface of the sheet 38 and at right angles to the normal undeflected direction of the beam.

The strips 31 which have a low value of electrical resistance are connected together in a zig-zag manner to form a series circuit by means of resistor elements 40 to 46. A voltage is connected between the ends 47 and 48 of the circuit. The elements 40 and 46 are graded so that the voltage drop is not linearly distributed over the length of the circuit, but provides a required distribution of the beam directing field, for example, an exponential distribution over the length of the screen in the undeflected direction of the beam. It is possible for the strips 39 to be proportioned, in such a way that they contribute to a required distribution of the field due to their resistance or lack of resistance. The strips 39 may be graded along their length with respect to resistance, for example by varying their widths, in order to provide a modification of the beam directing field pattern. The resistor elements 40 to 46 may be a resistive frit applied to the sheet 38, which may be of glass. Variations in the shape of the field may also be obtained by suitably shaping the strips 39 or by varying the pattern in which they are applied.

Referring to FIG. 6 there is illustrated diagramatically an arrangement of electrodes which enables two separate displays to be obtained, one on each side of a screen 50. The screen 50 provides a translucent support for a translucent conducting layer 51 and it has a luminescent phosphor layer 52 and 53 on respective opposite sides thereof. The arrangement includes two transparent beam directing electrodes 54 and 55 provided by transparent glass sheets coated on their inner surfaces with a transparent conducting layer and arranged one on each side of the screen 50. Two electron beams from sources not shown on opposite sides of the electrode assembly are indicated respectively by lines 56, 57 and 58, 59 at the extremes of their deflection. The screen 50 is tilted with respect to the electrodes 54 and 55 and beam directing fields are produced in the regions 60 and 61 between the electrode 54 and the screen 50 and the electrode 55 and the screen 50 which are of lower intensity nearer to their respective beam sources, due to the relative positions of the screen and the electrodes, when beam directing biassing potentials are applied to the electrodes 54 and 55 with respect to the screen. Employing this arrangement it is possible to provide two separate and independent displays by making the screen 50 opaque with a luminescent phosphor coating on each side so that displays of written information may be read off both sides of the screen independently. Such displays are of value in providing information to two parts of an area, for example in an airport lounge. A two colour display may be provided by making the screen 50 of FIG. 6 translucent and having on each side a phosphor coating which luminesces in a colour difierent from that of the other when it is bombarded by its respective electron beam.

Referring to FIG. 7 there is shown a diagrammatic illustration of a further arrangement for providing tow separate displays, one on each side of a screen 65, in which beam directing electrodes 66 and 67 are ar ranged parallel to one another and electron guns 68 and 69 having their respective deflection systems 70, 71 and 72, 73 are positioned at the same end of the structure to direct electron beams 74 and 75 into beam directing field regions 76 and 77 between the screen 65 and the electrodes 66 and 67 respectively. In a similar manner to that previously described with reference to FIGS. 1 and 2 the distribution of the static beam directing fields over the regions 76 and 77 varies in intensity linearly over the area of the screen in the directions of travel of the undeflected beams in order to act upon the beams 74 and 75 according to their angles of entry into the beam directing fields and to provide impingement of the beams on the screen in a required position. This arrangement may also be used to give either two separate displays or correlated displays in two different colours in a similar manner to that of FIG. 6.

Referring to FIG. 8 there is outlined an arrangement for providing either two separate black and white displays to be viewed independently from opposite sides or two correlated displays each in a different colour to be viewed together, which employs a single electron beam gun source 80. The gun source 80 provides a beam which passes via deflecting systems 81 and 82 either between a screen 83 and a beam directing electrode 84 on a path 85 or the screen 83 and a beam directing electrode 86 on a path 87. The beam may thus be switched between the path 85 and the path 87 to provide a two colour display with a modulation signal synchronised with the beam switching applied to the beam. Alternatively a beam splitting electrode may be positioned between the deflecting electrodes of the system 81 as indicated by the dotted line 88 in order to enable writing or scanning to take place on the opposite sides of the screen at the same time. In the arrangement shown the horizontal component of movement of the beams is provided in common by the deflection system 82. This embodiment is used preferably for colour displays viewed from one side only, however if information is to be written independently on the opposite sides of the screen and the two displays are to be viewed from the opposite sides of the screen, it must be remembered that on one side of the screen the beam moves from right to left while on the other side the beam moves from left to right and that the modulation signals applied to the beams must be co-ordinated with the movement of the beams in order that mirror writing by one of the beams is avoided.

In FIG. 9 there is shown a screen and beam directing electrode arrangement that may be used to provide a three colour display. In this arrangement a translucent screen 90 is provided with a serrated surface 91, the opposite surface 96 being planar. A phosphor 101 which luminesces in a first colour when bombarded by electrons is applied to the faces 92 of the serrated surface 91 and an electron beam 93 obtained from a first source (not shown) is directed to impinge upon the faces 92. A phosphor 102 which luminesces is a second colour when bombarded by electrons is applied to the faces 94 of the serrated surface 91 and an electron beam 95 obtained from a second source (not shown) is directed to impinge upon the faces 94. On the surface 96 on the other side of the screen 90 there is a conducting layer 100 upon which there is provided a third phosphor layer 103 which luminesces in a third colour. An electron beam 97 from a third source (not shown) is directed towards the third phosphor layer, The conducting layer 100 is arranged between two beam directing electrodes 98 and 99, at least one of which is transparent, in a manner similar to that of the previously described embodiments, and enables two beam directing fields to be provided. A uniform beam directing field is produced between the electrode 98 and the layer 100 and the faces 92 and 94 are so angled that the respective beams 93 and 95 may be caused to impinge upon them over the area of the surface 91 according to the direction in which the beams are launched into the beam directing field. A beam directing field, which increases in intensity in the direction of projection of the beam 97, is produced between the beam directing electrode 99 and the layer 100 to cause the beam 97 to be directed upon the third phosphor layer on the surface 96, in accordance with the direction in which the beam 97 is launched into the field. Co-ordinated scanning of the screen 90 by the electron beams 93, 95 and 97 may be used to produce a three colour display on the screen 90 which may be viewed through the transparent beam directing electrode.

An arrangement in which pairs of luminescent screens can be arranged to provide a tapered beam directing field between them will now be described with reference to FIG. 10, in which there are shown diagrammatically three luminescent screens 105, 106 and 107 on transparent supports and incorporating respective transparent electrically conducting electrodes. The screen 106 has a luminescent coating on each side. Electron beams are indicated at 108 and 109 projected into the spaces 110 and 111 between the electrodes of screens I and 106 and 106 and 107 respectively. In one mode of operation the electrode associated with the screen 106 is connected to a first potential, for example l kV and operated as a viewing screen and the electrodes 105 and 107 are connected to a second potential, for example 0.8kV, and operated as beam directing electrodes. Scanning is provided by horizontal and vertical deflection systems arranged outside the spaces 110 and 111 in the way previously described. An arrangement operated in the way described may be used to provide a two colour display by providing phosphors on the opposite sides of the screen 106 which luminesce in different colours and with the beams 108 and 109 impinging on the opposite sides of the screen 106, as shown. In a further mode of operation the potentials applied to the screen 106 and the beam directing electrodes 105 and 107 are modulated in such a way that, for example, for one line scan the screen 106 acts as a beam directing electrode directing the beams I08 and 109, as indicated by the dotted lines 112 and 113, towards luminescent coatings of still further colours on the screens 105 and 107, while for the next line scan the beams 108 and 109 are directed in the way originally described towards the opposite faces of the screen I06. In such a mode of operation, with each of the luminescent layers luminescing in a different colour, it is possible to provide multi-colour displays. If required a stack of such screens may be arranged and three dimensional displays may thus be provided. Such displays are of considerable value in showing the relative positions of moving objects, for example aircraft stacked above an airport. Of course, if desired, the phosphors may luminesce to provide black and white displays. To enable the invention to be more clearly understood a demountable experimental cathode ray tube assembly which has been successfully operated will now be described with reference to FIG. 11 which shows in a diagrammatic perspective view a standard 19 inch cathode ray tube face dish at 115, cut-away to disclose an electrode assembly and mounted and sealed upon a base plate 116 into which a vacuum pump tube I17 extends.

The electrode assembly for this device includes a standard cathode ray tube electron gun 118, a pair of horizontal deflection plates 119, a pair of vertical deflection plates 120, a luminescent screen 121 and a beam directing electrode 122. Provision is made in this experimental device for the connections (not shown) to the electrode assembly to be made via sealed terminals, as indicated at 123 and 124. The beam directing electrode 122 has a Nesa conducting coating and is operated as a single element monoplate, the intensity of the beam directing field being increased in the direction of emission of the electron beam from the gun 118 by inclining the electrode 122 towards the luminescent screen 121, so that they are closest at their edges remote from the gun. In this embodiment any corrections required to overcome aberrations or distortions of the electron beam are made electrically by applying correction potentials to the beam deflection and focussing electrodes in a way to be outlined below. The operation of the device, which has given acceptable pictures on the screen 121 of area approximately 3 inches by 4 inches, will be understood from the description of previous embodiments. If correction of the keystone effect is to be provided by the shaping of the deflecting electrodes it will be necessary to transpose the plates 119 and 120 so that the horizontal deflection plates 119, which will be suitably shaped, are arranged between the screen 121 and the beam directing electrode 122 and the vertical deflection plates 120. The vertical deflection plates 120 may, in such a case be considerably shortened.

In FIG. 12 a flat compact rectangular glass envelope 130, shown in section across its narrower dimension, contains a cathode ray tube assembly and has no projection to house the electron gun assembly as in the previously described embodiments. In this embodiment the housing is divided by a screen electrode 131, which shields the part of the envelope housing an electron gun assembly 132 from the part housing a display screen 133. An electron beam 134 passes from the gun assembly 132 via an accelerating electrode 135 and between a pair of diverging deflection plates, of which one is shown at 136, and which provide horizontal deflection of the beam, to an electron mirror or reversing lens having electrodes 137 to 140. Electron mirrors or reversing lenses are known for folding or reversing an electron beam obtained from a gun through l80 for impingement on a display screen arranged in a plane which is parallel to the axis of the gun. The electrodes 137 and 139 of the reversing lens system are curved in a known manner over their length in order to provide a field such that the beam 134 will leave the reversing lens system in a direction which is substantially parallel, though opposite to, the direction in which it left the gun assembly 132, whatever the deflection given to it by the horizontal deflection electrodes. The side electrodes 138 and 140 can also be shaped over their length as required to direct the beam as it leaves the reversing lens.

The display section of the cathode ray tube includes an electroluminescent screen 142 which comprises a transparent conducting layer and a coating of material which luminesces when bombarded by electrons supported on a transparent support screen. The conducting layer of the screen is connected to a terminal 141. Directly behind and spaced from the screen 142 there is a beam-directing electrode 143 which is parallel and co-extensive with the screen 142. The electrode 143 has a uniform coating of conducting material 144 connected between two strips of electrically conducting material 145 and 146. A terminal 147 on the envelope 130 is connected to the strip 145 and the strip 146 is connected via a lead 147a to a further terminal on the envelope not shown. The screen 131 is connected to a terminal 148 on the envelope.

Between the exit of the reversing lens and the space between the beam-directing electrode 143 and the screen 142, there is a pair of deflecting electrodes 150 which extends for substantially the width of the screen 142 and provides vertical deflection of the beam 134 over the screen 142 from the solid line position to the dotted line position 134a. The operation of this emodiment is similar to that for the previously described embodiment, with the exception that the reversing lens system is interposed between the horizontal and vertical deflection electrodes. The beam 134 is first accelerated by a positive potential on the electrode 135 between the horizontal deflection electrodes towards the relatively negative repeller electrode 137 of the reversing lens from which the collimated beam passes between the vertical deflection electrodes 150 into the beam-directing field between the screen 142 which is at final anode potential and the electrode 143. The beamdirecting field, as in previous embodiments, is arranged to increase in intensity in the direction of the beam away from the gun due to the direction of the connection of potential between the strips 145 and 146, and the linear distribution of the coating material 144.

In FIG. 13 a cathode ray tube assembly having the gun assembly located in an extension to one side of the main part of the envelope is shown. This arrangement uses a second beam-directing arrangement to direct the beam. The assembly of FIG. 13 includes an envelope 150 having a tubular side projection 151 housing an electron gun 152, and accelerating electrode 153, and a pair of deflection plates 154, 155 providing horizontal deflection of a beam 156 over a luminescent screen 157, as indicated by the solid line 156 and the dotted line 156a A beam directing field, which increases in intensity in the direction of the projection of the beam from the gun, is developed between electrodes 158 and 159. The electrode 159 has a longitudinal slot through which the beam 156 passes and potentials of the appropriate values to provide the field necessary to direct the beam in the way shown are connected between the electrodes 158 and 159. Potentials, applied between terminals 160 and 160a are distributed over the electrode by a uniform coating connected between strips 161 and 162 on the electrode 158.

After passing through the slot in the electrode 159, the beam 156 passes between a pair of deflector plates 163 to give vertical deflection of the beam before it enters a beam-directing field of the type previously described between the electroluminescent screen 157 and a beam-directing electrode behind and spaced from the screen 157. The screen 157 is electrically connected to a terminal 165 and the beam-directing electrode behind the screen has a uniform conducting coating between conducting strips 166 and 167 which are connected electrically to terminals 168 and 169 in order to provide a beam directing field in the space behind the screen 157 which increases in intensity in the vertical direction of scanning of the beam in the way previously described. In operation the beam 156 is scanned along the slot in the electrode 159 of the second beam directing arrangement under the influence of the horizontal beam deflection electrodes 154 and 155, thereby causing it to pass between the vertical deflection electrodes 163 at a position that determines its horizontal coordinate on the screen 157. Its vertical co-ordinate depends upon the amount by which it is deflected by the vertical deflection electrodes 163 in combination with the effect of the beam directing field in the space behind the screen 157.

In FIGS. 14 and 15 there is shown diagrammatically the essential features of a further practical experimental embodiment providing a monochrome display. The electrode assembly in this embodiment is housed in an envelope 170 consisting of two commercially available dished glass parts 171 and 172 and a tubular extension part 173 which extends into a semi-circular cut-out in each of the dished parts. The three parts are frit sealed in a well known manner after assembly of the electrodes.

The electrode assembly includes a conventional electron gun assembly having a cathode 174, a modulating or control electrode 175 and a focussing electrode assembly 176. A pair of horizontal deflection plates 177 and 178 is positioned at the mouth of the tubular part 173 and between these plates 177, 178 and a pair of vertical deflection plates 179, 180 there is an inter plate screen 181 having a longitudinal slot 182. A rectangular luminescent screen 183 of the type described with reference to FIG. 1 is positioned near to one of the inner surfaces of the envelope 170 and on the other surface of the envelope there is a transparent electrically conducting Nesa coating indicated at 184 covering a rectangular area corresponding to that of the screen 183. Terminals 195 and 186 are electrically connected to conducting strips along the edges of the coating 184 remote from and near to the gun assembly respectively so that the coating 184 can be connected to provide a beam-directing electrode. A terminal 187 is electrically connected to the screen 183. In the embodiment shown the screen 183 is approximately 4%inches square and the total length of the tube including the portion 173 is 10 inches. In operation the potential difference applied between the cathode 174 and the final anode terminal 187 connected to the screen 183 is 4 kV. The terminal 186 is connected to the same potential as the screen 183 and the terminal is connected to a potential which is 3 kV below that of the screen 183, thereby to provide a beam directing field in the space between the coating 184 and the screen 183 which has a greater intensity gradient remote from the electron gun assembly. An electron beam from the gun assembly is delfected by the plates 177, 178 to scan the slot 182 in the inter plate screen 81 and passes into the deflecting field of the plates 179, 180 where it is scanned orthogonally before it enters the field in the beam-directing space. The beam directing field directs the beam on to the screen 183 according to its angle and velocity of entry into the space between the screen and the coating 184 so that it scans the screen in a raster pattern. The deflecting fields are such that the raster pattern on the screen has the required 3:4 aspect ratio. The electrode 175 is modulated in the normal manner to provide a picture on the screen.

In FIG. 16 there is shown a circuit arrangement which enables a cathode ray tube beam to be controlled at a plurality of discrete positions as it is scanned over an electroluminescent screen and a parameter of the beam to be corrected for any abberation or distortion that might otherwise occur. The position of the beam is controlled by means of a co-ordinate signal output in binary form from a computer which establishes the co-ordinates of each of the plurality of positions of the beam on the electroluminescent screen. In the circuit in FIG. 16 the x and y co-ordinate outputs from the computer are applied in binary form via terminals 192 and 193, and in synchronism with a video signal applied to a terminal 194, to x and y registers 195 and 196 respectively. Each of these registers can be set to one of 256 different code combinations and a parallel output from each of the registers is applied to respective digital to analogue converters 197 and 198. Further outputs from these registers 195 and 196 are applied to respective x and y decoders 199 and 200. The digital to analogue converters 197 and 198 are capable of converting each of the 256 code combinations applied to them into discrete voltage levels and, with the code combinations fed to the registers in a natural binary sequence, the outputs from the converters 197 and 198 are scanning waveforms having a generally saw-tooth configuration with trace-defining portions formed of 256 discrete steps. The outputs from these converters 197 and 198 are applied via respective deflection amplifiers 201 and 202 to the x and y deflection electrodes ofa cathode ray tube 203. It is thus possible to identify 256 discrete deflection positions of the cathode ray tube beam in both the x and the y directions, and thus to identify 65,536 positions to which the beam can be deflected on the screen of the cathode ray tube. The x and y deflection signals cause the beam to scan from a co-ordinate position of X= and Y=O in the lower left hand corner of the screen, as seen by the viewer, from the left to the right and from the bottom to the top of the screen. In order to correct for trapezium distortion which results in the x scan near to the top of the screen remote from the gun being longer than the scan near to the bottom of the screen, a coupling circuit 204 is connected between the y deflection amplifier 202 and the digital to analogue converter 197. This coupling 204 is so porportioned that a part of the signal from the y deflection amplifier 202 is applied to the converter 197 and modulates the x deflection analogue signal output of the converter 197 according to the y deflection of the beam in order to correct the trapezium distortion. If a dc. coupled circuit is employed it is also necessary to connect an off-set" potential between the y deflection amplifier 202 and the x deflection amplifier 201 in order to correct what would otherwise be a parallelogram type scan into a rectangular scan.

It has been found that it is not necessary to perform the other corrections of the beam parameters at each of the co-ordinate positions and only a part of the binary signal from the x and y registers and 196 is applied to the decoders 199 and 200 so that, for example for every 256 changes of code in the registers 195 and 196 there are only 32 changes of code applied to the decoders 199 and 200. Furthermore, it has been found that it is only necessary to apply correction to the focussing of the beam in accordance with its scanning in the x direction and that movement of the beam in the y direction produces substantially no change in the focussing of the beam on the screen of the tube. However, correction for linearity and astigmatism are applied according to the scanning of the beam in both the x and the y directions. The signals for the correction of these parameters are applied to the respective x and y deflection amplifiers under the control of the outputs from the x and y decoders 199 and 200. In the circuit of FIG. 16 these corrections are made by applying, for each of the inputs to the x decoder 199, an output from the x decoder to an automatic focus correction circuit 205, a linearity correction circuit 206 and an astigmatism correction circuit 207 and for each of the inputs to the y decoder 200 an output from the y decoder to a linearity correction circuit 208 and an astigmatism correction circuit 209. These correction circuits are porportioned to provide a discrete output for each of the possible 32 inputs to the decoders in accordance with the distortions of the beam which iti is known would otherwise occur. Thus outputs from the focus correction unit circuit 205 are applied via an amplifier 210 to the focus control electrode of the cathode ray tube 205 and the outputs from the linearity and astigmatism correction circuit 206, 207 and 208, 209 are applied to their respective x and y deflection amplifiers 201 and 202.

It will be understood that with this form of control of the electron beam any co-ordinate position on the screen may be selected and addressed and the computer output may be programmed to direct the beam in other ways than to produce a raster scan, for example to produce alpha-numeric writing. Furthermore the corrections applied may be varied in accordance with the mechanical structure of the cathode ray tube. The circuit described has been developed to provide control for an experimental model of cathode ray tube and the need for certain of the corrections may be avoided by, for example, a different design of the electrode structure.

It is possible to operate cathode ray tubes in accodance with the present invention with a beam directing field of uniform intensity throughout the beam directing field space as may be seen from FIG. 9, however without some modification such as that shown to the screen in FIG. 9, the beam tends to become defocussed outside a limited area, although for some applications such an arrangement may be acceptable.

The bending of the beam in the beam directing field produces a self-focussing effect of the beam and little correction is, in fact, required. The effect of deflection focussing is well known in the operation of electrostatically deflected cathode ray tubes and it has previously been described in the literature, for example in the first mentioned of the Patent Specifications referred to above.

in its application to the production of a colour television image the tubes of the present invention have the advantage over tubes employing limited distributions of colour phosphors in the form of dots or stripes, that it is possible to obtain continuous straight lines in colour and thus stereoscopic effects which the known tubes are unable to reproduce.

The reversing lens arrangement of FIG. 12 may be provided by a combination of electromagnetic and electrostatic deflection systems.

Furthermore, although the cathode ray tubes described employ electrostatic deflection, it is possible to use electromagnetic deflection systems or a combination of the two. Similarly an electrostatic collimating lens structure may be employed in place of the magnetic lens 14 described with reference to FIGS. 1, 2 and 3. Furthermore, although the parts of the collimator lens 14 have been shown as rectangular members it is to be remembered that the illustrations are diagrammatic and that the parts of the lens may be of other suitable shapes, for example the pole pieces may be curved, in the view shown in FIG. 1, so that for example their central part is further from the gun 11 than their ends. As proviously mentioned, it is also possible to shape one of the deflection fields when using either electrostatic or electromagnetic deflection fields in order to provide a correction to the electron beam direction which compensates for the keystone distortion that would otherwise result. A degree of electrostatic collimation is naturally provided by virtue of the beam directing field and this must be taken into account when designing the collimating lens structure or the shaping of the deflection field to provide correction of the beam deflection. The collimating electrode system may involve the use of electrostatic transition electrode systems of the parallel plate type, alternatively a field mesh through which the beam passes may be positioned between the deflecting electrodes and the beam directing and screen system and held at a suitable potential.

Since a display must normally be viewed from only one side of the screen it is possible in most embodiments for the envelope of the assembly to have a transparent face on only one side and for the remainder of the envelope to be opaque and even of a different material. For example a construction is possible in which the transparent face is of glass and the remainder of the envelope is of metal. In such an arrangement the metal envelope may act as the beam directing electrode, being for example earthed while the screen and electron gun are at relatively positive and negative potentials.

However, as has been explained above it is possible to provide an envelope with viewing faces on both sides of the screen; and where there is a display on a transparent screen in such an envelope the view through one face will be a mirror image of that through the other. In some applications such a feature may not be objectionable. However, where two different displays are provided on the opposite faces of an opaque screen, the use of two separate guns and scanning systems enables two quite separate displays to be provided from normal signals. With an opaque screen and a single gun directing a beam on to a beam splitting electrode the modulating signal applied to the one of the split beams, in the case of a raster display, or one of the beam deflecting signlas in the case of a co-ordinate display, must be suitably co-ordinated if a mirror image display through one of the faces of the envelope is to be avoided. lf necesssary phosphors with a relatively slow decay time may be used to enable colour mixing to take place when providing multicolour displays. Arrangements providing displays in more than one colour are particularly advantageous in displays on aircraft control panels since it is possible to highlight a display of a parameter in a colour which attracts the eye should that parameter exceed a given limit.

A suitable beam directing field pattern between the screen and the beam directing electrode may be provided by forming the beam directing electrode so that it is not planar in order to give a variation in the field pattern over the area of the screen. Furthermore, the electrode and the screen need not be co-terminus in area, and the area covered by the electrode may be of a shape to produce required field effects. The beam directing electrode, although generally an electrically unitary structure may be made mechanically from a plurality of members connected electrically.

Although particular embodiments have been described modifications and combinations thereof may be made within the scope of the present invention.

In the embodiment of a beam directing electrode illustrated in FIG. 5 the conducting strips 39 are interconnected by resistors 40 to 46. A required field pattern could be produced by shaping the conducting strips, employing graded resistive strips instead of conductive strips or by providing a graded series of resistance coatings of different value between the strips 39. A further possibility, is that the intermediate strips 39 could be omitted and the graded series of coatings could contact one another, electrical connection to the assembly being made via the end strips 39. A linear or non-linear beam directing field may thus be provided as required.

It is also possible, in order suitably to shape the field to feed one or more potentials of different value to those at the ends to the beam directing electrode at separate positions along its length so that, where required, the strength of the beam directing field may be given a special value, for example an increase at these positions. The possibility also exists of varying the static beam directing field either linearly or non-linearly as required by varying the spacing between the screen and the beam directing electrode.

The beam directing electrode need not be a continuous layer it may be provided by a succession of parallel wires at right angles to the normal direction of the beam forming a grid. Each wire of such a grid could be supported between a pair of insulating supports and be connected to adjacent wires via resistors in the zigzag manner described with reference to FIG. 5, be connected to provide a plurality of sections, each fed separately with a suitable electric potential, or be part of a continuous length of wire of suitably resistive material to provide a beam directing field of linearly increasing intensity.

In embodiments in which it is required to write on one side of the screen only in order to produce a display it is possible for the screen to be formed on the face of the envelope of the tube.

With regard to the electrically conductive transparent coating described it is, of course, possible to use other materials than oxidised chromium.

ln the absence of the deflection of the beam 134 in the embodiment of FIG. 12, by either the horizontal on the vertical deflection electrodes 136, 150, the beam 134 enters the beam-deflecting field between the screen 142 and the electrode 143 in a direction which is parallel but opposite to that in which it was emitted from the gun.

In general the trajectory of the electron beam between the source and the screen is a continuous curve and there is no requirement for special means to maintain the beam parallel to the plane of the screen throughout any part of its trajectory.

From the foregoing description it may be seen that a flat cathode ray tube may be provided in which when an electron beam is launched from a gun into the space between a screen and a beam directing electrode at an angle relative to the normal direction of the beam determined by scanning means, the beam may be caused to impinge on a selected position on the screen according to the combined effect of a beam directing field distributed throughout the space and the angle at which the beam enters the space.

We claim:

I. Cathode ray tube apparatus including an electron beam source, an electroluminescent screen, a beamdirecting electrode spaced from the screen and including an insulating support sheet, a series of parallel strips of conductive material on the support sheet and resistor elements interconnecting the ends of the strips to provide a series connection; the electron beam source being arranged to launch an electron beam into the space between the screen and the beam-directing electrode, means to connect beam-directing potentials between the screen and the beam-directing electrode to provide a beam-directing field in the space between them for directing the electron beam towards the screen and means to defiect the beam in two directions arranged to be operative upon the beam before it enters the space.

2. Cathode ray tube apparatus according to claim 1 wherein the resistor elements are graded to provide a non linear distribution of voltage drop of a potential applied between the ends of the series circuit throughout the series circuit.

3. Cathode ray tube apparatus according to claim 1 having a second electroluminescent screen parallel to and luminescent in a colour different from that of the first mentioned electroluminescent screen, a second electron beam source and a second beam-directing electrode spaced from the second screen, the second electron beam source being arranged to launch an electron beam into the space between the second screen and the second beam-directing electrode, and means to connect beam-directing potentials between the second screen and the second beam-directing electrode to provide a beam-directing field in the space between them for directing the beam from the second source towards the second screen and second means to deflect the beam from the second source in two directions arranged to be operative upon the second beam before it enters the space between the second screen and the second beam directing electrode.

4. Cathode ray tube apparatus according to claim 1 having a second electroluminescent screen which is parallel to the first mentioned screen, a second beamdirecting electrode spaced from the second screen, and means to connect beam-directing potentials between the second screen and the second beam-directing electrode to provide a beam-directing field in the space between them for directing the electron beam towards the second screen, wherein the means to deflect the beam in two direction is arranged to deflect the beam in either the space between the first mentioned screen and beam-directing electrode or the space between second mentioned screen and beam-directing electrode.

5. Cathode ray tube apparatus according to claim 1 having a second electroluminescent screen parallel to and luminescent in a colour different from that of the first mentioned electroluminescent screen, a second electron beam source and a second beam-directing electrode spaced from the second screen, the second electron beam source being arranged to launch an electron beam into the space between the second screen and the second beam-directing electrode, and means to connect beam-directing potentials between the second screen and the second beam-directing electrode to provide a beam-directing field in the space between them for directing the beam from the second source towards the second screen and second means to deflect the beam from the second source in two directions arranged to be operative upon the second beam before it enters the space between the second screen and the second beam directing electrode.

6. cathode ray tube apparatus according to claim 5 including an electroluminescent screen support having a serrated surface with faces angled in two directions, wherein the second electroluminescent screen is formed upon the faces of the screen support which are angled in one direction, a third electroluminescent screen is formed upon the faces of the screen support which are angled in the other direction, a third electron beam source arranged to launch an electron beam into the field space between the support for the second and third electroluminescent screens and the second beam directing electrode to direct the electron beam therefrom towards the third electroluminescent screen, and third means to deflect the beam from the third source in two directions before it enters the space between the second beam directing electrode and the screen support for the second and third electroluminescent screens, the three luminescent screens each being luminescent in a different colour when bombarded by electrons.

7. A cathode ray tube according to claim 1 including an electron beam reversing lens arranged in the path of the beam of electrons between the electron gun and the said space, the arrangement of the electron gun, the reversing lens and the space being such that a beam of electrons which is undeflected by the said means for deflecting in passing from the gun through the reversing lens to the space enters the space in a direction which is parallel but opposite to that in which it was emitted from the gun.

8. A cathode ray tube according to claim 1 including a second beam-directing system arranged in the path of the beam between a first means for deflecting the beam in one of the orthogonal directions and a second means for deflecting the beam in the other orthogonal direction to change the direction of the beam. 

1. Cathode ray tube apparatus including an electron beam source, an electroluminescent screen, a beam-directing electrode spaced from the screen and including an insulating support sheet, a series of parallel strips of conductive material on the support sheet and resistor elements interconnecting the ends of the strips to provide a series connection; the electron beam source being arranged to launch an electron beam into the space between the screen and the beam-directing electrode, means to connect beam-directing potentials between the screen and the beamdirecting electrode to provide a beam-directing field in the space between them for directing the electron beam towards the screen and means to deflect the beam in two directions arranged to be operative upon the beam before it enters the space.
 2. Cathode ray tube apparatus according to claim 1 wherein the resistor elements are graded to provide a non linear distribution of voltage drop of a potential applied between the ends of the series circuit throughout the series circuit.
 3. Cathode ray tube apparatus according to claim 1 having a second electroluminescent screen parallel to and luminescent in a colour different from that of the first mentioned electroluminescent screen, a second electron beam source and a second beam-directing electrode spaced from the second screen, the second electron beam source being arranged to launch an electron beam into the space between the second screen and the second beam-directing electrode, and means to connect beam-directing potentials between the second screen and the second beam-directing electrode to provide a beam-directing field in the space between them for directing the beam from the second source towards the second screen and second means to deflect the beam from the second source in two directions arranged to be operative upon the second beam before it enters the space between the second screen and the second beam directing electrode.
 4. Cathode ray tube apparatus according to claim 1 having a second electroluminescent screen which is parallel to the first mentioned screen, a second beam-directing electrode spaced from the second screen, and means to connect beam-directing potentials between the second screen and the second beam-directing electrode to provide a beam-directing field in the space between them for directing the electron beam towards the second screen, wherein the means to deflect the beam in two direction is arranged to deflect the beam in either the space between the first mentioned screen and beam-directing electrode or the space between second mentioned screen and beam-directing electrode.
 5. Cathode ray tube apparatus according to claim 1 having a second electroluminescent screen parallel to and luminescent in a colour different from that of the first mentioned electroluminescent screen, a second electron beam source and a second beam-directing electrode spaced from the second screen, the second electron beam source being arranged to launch an electron beam into the space between the second screen and the second beam-directing electrode, and means to connect beam-directing potentials between the second screen and the second beam-directing electrode to provide a beam-directing field in the space between them for directing the beam from the second source towards the second screen and second means to deflect the beam from the second source in two directions arranged to be operative upon the second beam before it enters the space between the second screen and the second beam directing electrode.
 6. cathode ray tube apparatus according to claim 5 including an electroluminescent screen support having a serrated surface with faces angled in two directions, wherein the second electroluminescent screen is formed upon the faces of the screen support which are angled in one direction, a third electroluminescent screen is formed upon the faces of the screen support which are angled in the other direction, a third electron beam source arranged to launch an electron beam into the field space between the support for the second and third electroluminescent screens and the second beam directing electrode to direct the electron beam therefrom towards the third electroluminescent screen, and third means to deflect the beam from the third source in two directions before it enters the space between the second beam directing electrode and the screen support for the second and third electroluminescent screens, the three luminescent screens each being luminescent in a different colour when bombarded by electrons.
 7. A cathode ray tube according to claim 1 including an electron beam reversing lens arranged in the path of the beam of electrons between the electron gun and the said space, the arrangement of the electron gun, the reversing lens and the space being such that a beam of electrons which is undeflected by the said means for deflecting in passing from the gun through the reversing lens to the space enters the space in a direction which is parallel but opposite to that in which it was emitted from the gun.
 8. A cathode ray tube according to claim 1 including a second beam-directing system arranged in the path of the beam between a first means for deflecting the beam in one of the orthogonal directions and a second means for deflecting the beam in the other orthogonal direction to change the direction of the beam. 